In a portable telephone with a build-in digital camera, an imaging lens is mounted on a printed circuit board. As a method for mounting an imaging lens on a printed circuit board, a reflow soldering processing is used. Hereafter the reflow soldering processing may simply be called “reflow”. Reflow processing is a method for soldering an electronic component by placing a solder ball in advance at a location where an electronic component is connected, placing the electronic component there, heating to melt the solder ball, then cooling the solder down.
Generally in mass production steps, a reflow step for performing reflow processing is used as a method for mounting electronic elements or such components as an imaging lens on a printed circuit board. If the reflow step is used, the mounting cost of components on a printed circuit board can be decreased, and the manufacturing quality can be maintained at a predetermined level.
In the reflow step of the manufacturing steps of a portable telephone comprising an imaging lens, not only electronic components are arranged at predetermined positions on a printed circuit board, but also the imaging lens itself and a socket for installing the imaging lens are arranged on the printed circuit board.
The imaging lenses installed in portable telephones are largely made of plastic in order to decrease manufacturing cost and to insure lens performance. Therefore a heat resistant socket component is used for installing the imaging lens, in order to prevent thermal deformation of the imaging lens in a high temperature environment, which makes it impossible to maintain optical performance thereof.
In other words, in the reflow step, a heat resistant socket component for installing an imaging lens is mounted on the printed circuit board of the portable telephone, and the imaging lens is installed in this socket after the reflow step, so that the imaging lens is not exposed to the high temperature in the reflow step (e.g. see Patent Documents 1 to 3). However using a heat resistant socket component for installing the imaging lens makes the manufacturing steps complicated, and increases the manufacturing cost including the cost of this heat resistant socket.
Recently it has been demanded that the optical performance of an imaging lens installed in a portable telephone does not deteriorate even if the portable telephone itself is placed in 150° C. or higher temperature environment, considering the case of a portable telephone that is left in an automobile which temporarily becomes a high temperature environment. A conventional imaging lens made of plastic material cannot meet this demand.
In order to implement an imaging lens of which optical performance is maintained even in a high temperature environment, forming an imaging lens using a high melting point mold glass material is possible (e.g. see Patent Document 4). According to this, the deterioration of optical performance of an imaging lens in a high temperature environment can be avoided, but at this moment, an imaging lens made of mold glass material is not popular since the manufacturing cost is very high.
In addition to the above thermal characteristics, an imaging lens installed on a portable telephone must satisfy the following conditions related to optical characteristics. One condition is that the optical length, which is defined as a length from an entrance plane at an object side to an image formation plane (also called the “image sensing plane”) of the imaging lens, must be short. In other words, when a lens is designed, the ratio of the optical length to the composite focal distance of the imaging lens must be minimized.
In the case of a portable telephone, for example, this optical length must at least be shorter than the thickness of the portable telephone main unit. On the other hand, a back focus, which is defined as a distance from the outgoing plane at the image side to the image sensing plane of the imaging lens, should be as long as possible. In other words, when the lens is designed, the ratio of the back focus to the focal distance must be maximized. This is because such components as a filter and cover glass must be inserted between the imaging lens and the image sensing plane.
In addition to this, it is naturally demanded for the imaging lens that various aberrations are corrected to be small enough that the distortion of an image is not visually recognized, and the integration density of the image sensing elements (also called “pixels”) is sufficiently satisfied. In other words, various aberrations must be well corrected, and an image when various aberrations are well corrected may hereafter be called a “good image”.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-121079    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-328474    Patent Document 3: Japanese Patent No. 3755149    Patent Document 4: Japanese Patent Application Laid-Open No. 2005-067999